High end storage controllers, such as the International Business Machines Corporation (IBM®) Enterprise Storage Server® manage Input/Output (I/O) requests from networked hosts to one or more storage units, such as a direct access storage device (DASD), Redundant Array of Independent Disks (RAID Array), and Just a Bunch of Disks (JBOD). Storage controllers include one or more host bus adaptors or interfaces to communicate with one or more hosts over a network and adaptors or interfaces to communicate with the storage units. The storage controllers also include a cache memory and often further include a non-volatile storage device (NVS) which generally has a substantially smaller storage space than the cache memory) and which may be comprised of a battery backed-up random access memory. In a Fast Write operation, the storage controller receives write requests from the host device and writes data updates to both the cache and NVS. The NVS is used to provide an extra copy of the data update in the event that the copy of the update in cache is lost as a result of a system failure. The storage controller returns a “write complete” message to the host which initiated the update request upon completing the write to both the cache and NVS, but before the data is actually written to the storage unit. The cached data is subsequently destaged to a target storage unit. Once destaged to disk, the update copy in the cache and NVS may be removed. The storage units may be configured as logical devices or “ranks” making the physical devices transparent to the host device. The cache and NVS unit of a storage controller may store updates intended for multiple ranks and each rank may be allocated a percentage of the total capacity of the NVS (see, for example, co-pending and commonly assigned U.S. patent application Ser. No. 2003/0037,204 now U.S. Pat. No. 6,775,738, filed Aug.17, 2001 and entitled Method, System, and Program for Caching Data in a Storage Controller, which application is incorporated herein by reference). If a rank dominating the NVS is processing I/Os at a slow rate, then other processes submitting I/Os to other ranks are delayed such that updates are destaged at the rate of the slower, dominating rank. Moreover, if the NVS is destaging updates at a rate slower than it is able to copy new updates, a backlog is created and new write requests may have to be refused. Thus, because “complete” status is not returned to the requesting host unless the update is copied to both the cache and the NVS, a slowdown by a rank may cause a backlog in the NVS processing write requests, ultimately resulting in a slowdown in the host through-put and possible unfavorable host time-out conditions.
One method developed by IBM for reducing the effects of an NVS (or cache) backlog is, upon detection of a backlog (determined by comparison to a predetermined threshold rate), to reduce the rate at which write requests are transmitted from the host relative to the rate at which data updates are destaged. Such a “throttling” may be to a fixed, slower ratio of write requests per destage. However, other conditions may be present which alleviate the need for throttling but which are not taken into account when the reduced ratio is imposed.
Consequently, a need remains for autonomically or dynamically modifying the destage ratio depending upon workload conditions.